My understanding is that for the EPA to delegate Clean Air Act responsibilities, the State must submit a SIP (State Implementation Plan) that gets reviewed and approved by EPA. I concur that I doubt that Davis has gone to the trouble of running a change in the SIP throught the state & federal regulatory review process, because of all the bad political fallout from what might happen. As such the state has a filed program with EPA that probably uses MTBE as a major element in meeting certain criteria pollutant limits. And now they just want to magically change what they are doing without going throught the process. If they go through the process, a lot of business interests will want to lighten up and change other things (always dicey from and environmentalist standpoint in an election year when campaign contributions are so appreciated.) This should be very interesting to watch.
I do know that legislated mandates are generally the poorest way to enforce policies of an essentially technical nature. In MBTE's case, it is mandated for reducing smog producing emissions, but if new info comes to light that it is ineffective in that use, or that that it causes more trouble than it solves (the critics argument), that info doesn't matter because it was specified in the law.
I have a similar problem in NC where public swimming pool regs specify a chlorine residual concentration by legislation, even though recent advances in equipment and technology make ozone a superior choice for swimming pool treatment. But I can't sell such systems here because the law specifically requires chlorine instead of requiring standards for disinfection efficacy simply because that legislation would be over the intellectual heads of most legislators.